The present invention relates to a light emitting semiconductor element for emitting green or blue light.
Optical information technology has lately developed very rapidly with the increased production of laser discs and/or laser printers. Those devices have a semiconductor laser which emits red light as a light source. In order to achieve high density recording and/or high speed printing, the light source must be not only of a high power, but also short in wavelength. Although a short wavelength semiconductor has been proposed, no semiconductor laser which emits green or blue light (wavelength is 0.3-0.6 micron) has yet been obtained. Additionally, a blue light emitting diode is necessary for providing full color display by using light emitting diodes, however, no such diode has been obtained.
As for a short wavelength light emitting semiconductor element, in particular, green and/or blue light, because of the large energy gap of direct transition, the II-VI group compound semiconductor like ZnS, ZnSe is promising.
Further, it is recognized that Zn type II-VI group compound semiconductor is useful as a secondary harmonics generator element, and/or an optical flip-flop, and a new function element in general.
FIG. 4 shows the relations between the energy gap (horizontal axis) and the lattice constant (vertical axis) of II-VI group compound semiconductor (ZnS, ZnSe, CdS, CdSe, ZnTe, CdTe), III-V group compound semiconductor (GaAs, GaP, InP), and IV group semiconductor (Ge).
However, the Zn type II-VI group compound semiconductor has the disadvantage that no excellent crystal grows on a III-V group compound semiconductor substrate (for instance GaAs). The reason of that is that the growing layer is distorted due to the temperature change, because of the difference of the linear thermal expansion coefficient between the growing layer and the substrate.
FIG. 5 shows the relations between the temperature and the lattice constant of a prior compound semiconductor, in which the characteristics of ZnS.sub.0.06 Se.sub.0.94 (atomic ratio) which lattic-matches with GaAs substrate at room temperature, ZnS.sub.0.09 Se.sub.0.91 which lattice matches with GaAs substrate at 500 .degree. C., and GaAs are shown. It is noted in the figure that ZnS.sub.0.06 Se.sub.0.94 which lattice matches with GaAs at room temperature, does not lattice match with the active layer and the clad layers at 500.degree. C., and the lattice mismatching error is 0.13% at 500.degree. C., which is the typical growing temperature. Therefore, if ZnS.sub.0.06 Se.sub.0.94 is grown on GaAs substrate at 400.degree.-600.degree. C., the distortion is generated in the growing process, and the lattice defect and/or the impurity diffusion from the substrate occur.
On the other hand, in case of ZnS.sub.0.09 Se.sub.0.91 ' although it lattice matches with GaAs substrate at the growing temperature, it does not match at room temperature, and the lattice mismatching is 0.13% at room temperature. Therefore, when the temperature is decreased, the growing layer is distorted, and the lattice defect, and/or the impurity diffusion from the substrate occur.
As mentioned above, when Zn type II-VI group compound semiconductor is grown on III-V group compound semiconductor substrate like GaAs, GaP, InP, or IV group substrate like Ge, Si, the growing layer is distorted because of the temperature change, and a poor growing layer is obtained. Thus, an excellent light emitting device for short wavelengths has not to be obtained.